Our classroom is -----（在顶楼）of Building A.这句话应该用 on the top f

正确的介词是 on ,比如
on the first floor
on the second florr
on the top floor
正确的表达方式是
Our classroom is (on the top floor) of Building A.
我是加拿大人,前英语老师.
英语是我第一语言.